1. Field of the Disclosure
The present disclosure relates to a Liquid Crystal Display (LCD) device, and more particularly, to an array substrate for a fringe field switching mode liquid crystal display device, which enables measurement of the properties of thin film transistors included in a display area of the display device.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device is driven using optical anisotropy and polarization of liquid crystal molecules. Since liquid crystal molecules have a thin, long molecular structure, an arrangement of the liquid crystal molecules has directivity. Thus, when an electric field is applied to the liquid crystals, the molecular arrangement direction of the liquid crystals is changed.
Accordingly, by arbitrarily adjusting the molecular arrangement direction of the liquid crystals, the molecular arrangement of the liquid crystals is changed, and light is refracted in the molecular arrangement direction of the liquid crystals to thereby display image information.
An active matrix LCD (AM-LCD, hereinafter simply referred to as an LCD) in which thin film transistors and pixel electrodes connected to the thin film transistors are arranged in a matrix form has been attracting attention due to its high resolution and excellent quality of fast moving images.
The LCD generally includes a color filter substrate in which a common electrode is formed, an array substrate in which pixel electrodes are formed, and liquid crystals filled between the color filter substrate and the array substrate. The common electrodes and the pixel electrodes drive liquid crystals by an electric field applied in an up-down direction therebetween. The LCD has an excellent light transmission rate and aperture ratio.
However, in the general LCD, liquid crystal driving by an electric field applied in the up-down direction becomes a factor for deteriorating a viewing angle property. In order to overcome this disadvantage, an in-plane switching mode LCD having an excellent viewing angle has been proposed.
Hereinafter, a general in-plane switching mode LCD will be described in detail with reference to FIG. 1.
FIG. 1 is a cross-sectional view of the general in-plane switching mode LCD.
As shown in FIG. 1, an upper substrate 9 which is a color substrate is spaced apart from a lower substrate 10 which is an array substrate, and faces the lower substrate 10. A liquid crystal layer 11 is inserted between the upper and lower substrates 9 and 10.
On the lower substrate 10, a common electrode 17 and a pixel electrode 30 are formed on the same surface, and the liquid crystal layer 11 is driven by a horizontal electric field L formed by the common electrode 17 and the pixel electrode 30.
FIGS. 2A and 2B are views for explaining operations when the in-plane switching mode LCD of FIG. 1 is turned on and off, respectively.
First, referring to FIG. 2A showing an arrangement of liquid crystal molecules when a voltage is applied to the in-plane switching mode LCD, the arrangement of liquid crystal molecules 11a positioned at locations corresponding to or directly above the common electrode 17 and the pixel electrode 30 does not change, whereas liquid crystal molecules 11b positioned between the common electrode 17 and the pixel electrode 30 are arranged by a horizontal electric field L formed by the common electrode 17 and the pixel electrode 30 in the direction of the horizontal electric field L. In other words, the in-plane switching mode LCD can achieve a wide viewing angle since the liquid crystals are moved by the horizontal electric field.
Therefore, images can be viewed even at angles of about 80° to 89° with respect to the front of the in-plane switching mode LCD, without causing image inversions.
Then, referring to FIG. 2B showing an arrangement of liquid crystal molecules when no voltage is applied to the in-plane switching mode LCD so that the in-plane switching mode LCD is turned off, since no horizontal electric field is formed between the common electrode 17 and the pixel electrode 30, the arrangement of the liquid crystal molecules 11 does not change.
The in-plane switching mode LCD, however, has a low transmission rate and aperture ratio although it can provide an improved viewing angle. In order to overcome these disadvantages of the in-plane switching mode LCD, a fringe field switching mode LCD in which liquid crystals are driven by a fringe field has been proposed.
FIG. 3 is a cross-sectional view of an array substrate 41 for a fringe field switching mode LCD according to a related art, cut along a line crossing the center portion of a pixel area of the LCD.
As shown in FIG. 3, the array substrate 41 for the fringe field switching mode LCD includes a plurality of pixel areas P, a gate line (not shown), and a data line 47, wherein the plurality of pixel areas P are defined by intersections of the upper parts and the lower parts with a gate insulating film 45 therebetween. In each pixel area, a thin film transistor Tr connected to the gate line and the data line 47 is formed. Also, in each pixel area on the gate insulating film 45, a plate-shaped pixel electrode 55 is formed to contact the drain electrode 51 of the thin film transistor Tr. The pixel electrode 55 is formed on the same layer as the data line 47, that is, on the gate insulating film 45, and is spaced a predetermined distance from the data line 47 in order to prevent a short with the data line 47.
Also, a protection layer 60 is formed on the data line 47 and the pixel electrode 55 throughout the entire display area, and a common electrode 65 is formed on the entire surface of the protection layer 60. The common electrode 65 has a plurality of bar-shaped openings oa, which are spaced a predetermined distance from each other in correspondence with the individual pixel areas.
The array substrate 41 for the fringe field switching mode LCD, as described above, should be tested to determine if each thin film transistor Tr has any defect and to check the properties of the thin film transistor Tr. However, since the common electrodes 65 are formed throughout the entire display area, it is not possible to check the properties of the thin film transistor Tr included in the pixel areas configuring the display area of the LCD. Further, it is not possible to check whether or not a corresponding gate or data line connected to a plurality of the thin film transistors Tr's is properly functioning or formed.
More specifically, in order to check the properties of the thin film transistor Tr included in each pixel area of the array substrate 41, a gate signal voltage and a data signal voltage need to be supplied to the gate electrode 43 and the source electrode 49, respectively, to drive the thin film transistor Tr, and also a signal voltage or current changing according to a data signal voltage supplied through the drain electrode 51 needs to be output.
However, in the array substrate 41 for the fringe field switching mode LCD according to the related art, as described above, since the drain electrode 51 of the thin film transistor Tr is connected to the pixel electrode 55 and the pixel electrode 55 is covered by the protection layer 60 and the common electrode 65, it is not possible to make a contact with the drain electrode 51 or with the pixel electrode 55 contacting the drain electrode 51 even when the data signal and the gate signal are respectively applied through the data line 47 and the gate line connected to the thin film transistor Tr. As a result, it is not possible to measure outputs from the thin film transistor TR formed in each pixel area P included in the display area of the LCD.
Accordingly, in the case of the array substrate 41 for the fringe field switching mode LCD according to the related art, a test for checking the properties of the thin film transistor Tr is not possible due to the structural configuration of the LCD.